Curable formulations for adhesive, coating, molding and potting applications based on thiol-ene chemistry are well known. A detailed discussion of the general background art in this field may be found in U.S. Pat. 4,808,638 and in the cited references thereto.
This invention relates to a particular subgroup of thiolene formulations in which the thiol and ene are both silicones and the ene resin is an acrylic group.
As used herein, the term "silicone" is used in its conventional sense to refer to polyorganosiloxane polymers. Typically the organo groups are alkyl, aryl, or haloalkyl. Examples of such groups include methyl, ethyl, phenyl and trifluropropyl. Other organo groups may also be present. The term "(meth)acryl" is used generally to refer to both acryl and methacryl functional groups. The term "acrylic" is used generally to refer to both acrylate and methacrylate groups unless the context indicates otherwise.
Thiol-ene formulations employing organo acrylic ene compounds are described in U.S. Pat. 4,008,341; U.S. 4,120,721 and in Gush et al, "Thiol/Acrylate Hybrid Systems in Radiation-Curable Coatings--The Best of Both Worlds", presented at the NPCA Chemical Coatings Conference II, Cincinnati, Ohio, May 10, 1978.
Curable (meth)acryloxy or (meth)acrylamide functional silicone formulations which also employ organothiol functional silicones, as radical chain transfer agents or for thiol-ene co-curing, are described in US 4,290,869; US 4,595,471; and EP 273,565. None of these references describe formulations which utilize alkenylene linking groups between the (meth)acrylic functional group and the silicone backbone.
In U.S. 4,503,208, 4,504,629, 4,575,545, 4,575,546 and 4,760,122 all of which are incorporated here in by reference, the preparation of (meth)acryloxyalkenylene functional silicones is described. Such silicones have a plurality of groups of the formula: ##STR2## attached to silicon atoms thereof where R is H or methyl, and R.sup.1 is a divalent olefinically unsaturated hydrocarbon group. These references do not describe or suggest curing the resins by thiolene addition reactions.
Researchers at Dow Corning ,Company, a company which has extensively investigated thiol-ene curing of vinyl silicones, have also reported that vinyl functional silicones cured by the thiol-ene reaction display much poorer thermal stability properties compared to the properties of vinyl silicones cured without polythiol. This result has been explained as "undoubtedly a result of the monosulfide crosslink." Clark, et al, "Ultraviolet Curable Silicone Elastomer Useful as Optical Fiber Coating", Polym, Mater, Sci. Eng., 1985, 52 442-47. Other Dow Corning Company researchers have also recently reported "mercapto-olefin functional siloxanes crosslink extremely fast by radiation and curing is not inhibited by oxygen. These systems, however, have several inherent drawbacks such as obnoxious odor and thermal instability" (emphasis added). P.J. Varaparth, et al., RadTech Proceedings, RadTech International, Northbrook, Illinois, pp 16-29 - 16-38 (April 1988).